FIG. 1 is a graph of the output voltage of a typical antenna and front end circuit. In this common configuration, the antenna produces a voltage when excited by an electromagnetic field. This voltage is commonly called the open-circuit voltage across the antenna terminals. When the antenna terminals are connected to a front end circuit, power is transferred from the electromagnetic field through the antenna and into the front end circuits (front end). Front ends are generally known in the art and are used to convert (or down convert) the AC electromagnetic field into an intermediate frequency (IF) or direct current (DC) frequency.
Front end and antenna combinations have various designs depending on the application that the design is to perform. To illustrate this, FIG. 1 shows the voltage output of a front end and antenna combination versus frequency of the electromagnetic field. This voltage output has two regions: 1. a flat region 110 over a wide range of frequencies that produces a relatively low voltage output, and 2. a resonant region or bandwidth 120 centered about a resonant frequency 125 where the antenna produces a relatively large voltage over a smaller frequency range.
In some applications, e.g., field sensors, the antenna/front end combination is designed to disturb an electromagnetic field as little as possible. A field sensor measures the strength of an electromagnetic field and typically uses small antennas that operate over the wide frequency band 110, i.e., not around a resonant frequency 125 of the field sensor antenna. Over the range of frequencies 110, the front end is tuned so that it is out of resonance with the antenna. Therefore, there is a minimum of power taken by the combination, i.e., there is a minimum of power transferred from the antenna to the front end. Another way of stating this is that the antenna is loaded with a mismatched load (front end) that limits how much the electromagnetic field can excite the antenna. In this type of application, the combination is equally sensitive over a wide frequency range 110 and draws a minimum amount of power from the field, i.e., the sensor perturbs the field a minimum amount. In these applications, the antenna resonant frequency is chosen to be well outside the operation frequency range 110 and the front end is designed so that the combination does not resonate in the operation frequency range 110.
In other applications, antennas operate over the bandwidth 120 to receive/transmit signals over as wide a bandwidth as required. Generally, the bandwidth 120 of the antenna is relatively narrow but is widened in some cases, e.g., in television, radio, and some radar systems, to transmit/receive over a large number of channels or over a wide continuous spectrum. In other applications, e.g., those where a narrow bandwidth is required by law, antenna designers narrow the bandwidth 120 as needed to comply with the requirements. In these applications, the front end is designed to resonate with the antenna over the operation frequency range 120 so that the maximum amount of power is transferred between the antenna (and hence the electromagnetic field) and the front end (and hence any circuitry attached to the front end). In many embodiments of this type, the front end is variably tunable over a plurality of frequencies 125 so that the operation frequency range 120 varies over the frequency scale 130.
In the particular field of radio frequency identification (RFID) tags, especially passive RFID tags, antennas connected to the front end and the rest of the RFID circuit need to produce a front end output voltage that is above a threshold voltage, in order to power the RFID circuit. This is typically accomplished by trying to match the antenna impedance to that of the front end of the RFID circuit (e.g. a chip) at the resonance frequency 125. These front end circuits typically use diode and capacitor circuits (the front end) that rectify the radio frequency (RF) carrier component of the modulated electromagnetic field, that excites the antenna, leaving the modulated signal (envelope) at the output of the front end.